1. Field of the Invention
The invention relates in general to the poling of a body of polymeric piezoelectric material and more particularly, to the poling of such material to produce nonuniform polarization across the thickness of the body to render the body excitable in a flexure mode.
2. Discussion Related to the Problem
A piezoelectric flexure mode device comprises a plurality of layers of material of differing piezoelectric activity. Under the application of an external field across the thickness of such a device, small expansions and/or contractions in the planes of opposing layers are converted into relatively large deflections of the layers out of their planes. Alternatively, if an external force is applied to the device causing it to flex, a voltage is generated across the layers of the device. Piezoelectric flexure mode devices have found utility both as electrical-to-mechanical and mechanical-to-electrical transducers in such diverse applications as speakers, microphones, phonograph cartridges, motors, and accelerometers. In its simplest form, a piezoelectric flexure device comprises a two layer structure, one of the layers exhibiting piezoelectric activity and the other not. When an electric field is applied across the thickness of the layers, the piezoelectric layer expands or contracts and because the other layer, which is not piezoelectrically active, resists such expansion or contraction, the device flexes or bends. This form of flexure mode device is known as a unimorph. A more common form of flexure mode device, known as a bimorph, comprises two layers of piezoelectric material arranged so that when one expands, the other contracts resulting in approximately twice the deflection for a given applied field. Conventionally, piezoelectric flexure mode devices are manufactured by separately uniformly poling sheets of piezoelectric material, then bonding the sheets together in the described configuration, using an adhesive. This technique has long been employed for making flexure mode devices from piezoelectric ceramic materials such as lead zirconate titanate (PZT). For more information on the structural details of such devices and some of their various uses, one may refer to the article by C. P. Germano entitled "Flexure Mode Piezoelectric Transducers", IEEE Transactions on Audio and Electroacoustics, Vol. AU-19, No. 1, March 1971.
More recently, polymeric piezoelectric materials, such as polyvinylidene fluoride (PVF.sub.2), have received considerable attention. As plastics, they are very attractive, since well developed conventional plastic manufacturing technologies may be adapted to their economical manufacture. One of the major problem areas encountered in making polymeric piezoelectric flexure mode devices has been in the bonding of layers to form the devices. Problems with adhesion of the bonding material, and proper matching of the mechanical impedance of the adhesive and the polymeric piezoelectric material have been encountered. One way of overcoming the bonding problem in the manufacture of a piezoelectric ceramic flexure mode device is taught in U.S. Pat. No. 2,659,829 issued Nov. 17, 1953 to H. G. Baerwald. Baerwald discloses the technique of nonuniformly poling a single sheet of piezoelectric material so that it behaves like a unimorph or bimorph. According to the technique, a slab of piezoelectric material is first poled uniformly in one direction, then the poling is relaxed in a portion of the thickness of the slab by propogating a thermal pulse to a limited depth that momentarily raises the temperature of the material above the Curie point. The resulting structure comprises a continuous slab of material having layers of different piezoelectric activity within the continuous slab. It resembles a monomorph, with one layer piezoelectrically active and another layer substantially inactive. In a further embodiment, the slab is then subjected to a poling field of the opposite polarity and of such a strength so as to polarize the unpoled material but not to reverse the polarity of the already poled portion of the material. The result is a device similar to a bimorph, having two layers poled in opposite directions in one continuous slab of material.
If this technique could be applied to polymeric piezoelectric materials to produce a multilayer structure in a continuous body of material, the bonding problems could likewise be resolved. Unfortunately, polymeric piezoelectric materials such as PVF.sub.2 do not exhibit a true Curie point below the melting point of the polymer. When the thermal depolarization technique is attempted, all that results is a molten blob of plastic.
Some observers have noted that single films of PVF.sub.2, poled under certain conditions of electric field and temperature, exhibit flexure mode behavior attributable to a nonuniform distribution of piezoelectric activity across the thickness of the film (see the article by H. Sussner and K. Dransfeld entitled "Importance of the Metal-Polymer Interface for the Piezoelectricity of Polyvinylidene Fluoride", Journal of Polymer Science: Polymer Physics Edition, Vol. 16, 529-543, (1978), John Whiley & Sons, Inc.). The nonuniformly poled polymeric piezoelectric material produced thereby, although exhibiting some flexure mode behavior, is not nearly as active in flexure mode as the devices produced by bonding multiple layers of differently poled material. It is a purpose of the present invention to provide methods of nonuniformly poling polymeric piezoelectric material, such as PVF.sub.2 that increase the flexure mode activity over that observed in the prior art.